The present invention relates to a device for receiving the exhaust gas output of a reciprocating piston internal combustion engine, and more particularly discloses a muffler assembly that is particularly advantageous for use with a two-stroke gasoline engine, which assembly provides both an exhaust scavenging function and a supercharging function, and in addition, comprises a low profile and compact design.
This invention discloses an exhaust gas handling assembly for an internal combustion engine, which is especially useful in small two-stroke gasoline engines such as in radio-controlled airplanes and wheeled vehicles for ground travel, such as motorcycles and all terrain vehicles. The device is commonly referred to as a muffler, but this is a term that is too restrictive for all the functions performed by the device. While the device does serve to muffle or dampen the noise of combustion in such internal combustion engines, it also serves at least two other critical functions, exhaust scavenging and fuel-charge densification.
The present invention has been found to be particularly advantageous when used on a two-stroke, internal combustion, piston and crankshaft type engine which burns a volatile fuel such as gasoline and/or alcohol, and which utilizes valving consisting of ports formed through the wall of the piston cylinder, controlled by movement of the piston within the cylinder to alternately expose and cover up said ports.
A typical two-stroke engine has one or more intake ports formed through each cylinder wall and one or more exhaust ports formed through the cylinder wall, usually located on the opposite side of the cylinder from the intake ports. These ports are positioned such that the piston opens and closes them in a carefully controlled sequential manner to allow intake and exhaust of the fuel/air mixture and the products of combustion, respectively. Many such engines pump the fuel/air mixture through the crankcase of the engine into the intake port in the cylinder wall.
During a normal intake/compression/combustion/exhaust cycle of the two-stroke piston-cylinder combination, when the exhaust port is opened by movement of the piston away from its blocking position over the port, a high-pressure exhaust gas pulse starts down the exhaust tube. The piston continues down and the exhaust pressure bleeds off into the tube. This occurs at around 90-110 degrees from piston Top Dead Center (TDC). At about 15-25 degrees later, the intake ports on the other side of the cylinder are exposed by the piston, and, because of crankcase compression, a fuel/air mixture begins to flow through the intake ports and into the cylinder while exhaust gas is still moving out the exhaust ports. After a small fraction of a second, the pressure pulse moving down the exhaust tube reaches an open area, or expansion chamber, and this starts an expansion wave back toward the exhaust ports. This expansion wave creates an action at the exhaust ports, which serves to draw additional flow of exhaust from the cylinder, including a portion of the new fuel/air charge entering through the intake ports.
As the expanding exhaust pulse reaches the end of the expansion chamber, it impinges the narrowed end of the tube at the downstream end of the chamber and is compressed, thereby creating a strong compression wave that moves back up the tube to the exhaust port. This results in some of the escaped fuel/air charge being pushed back into the cylinder before the piston closes the exhaust ports, thus achieving the desired charge-densification effect in the cylinder.
The xe2x80x9ctuningxe2x80x9d of the muffler is dependent upon the length and volume of the expansion chamber and its distance down the tube from the exhaust ports. This chamber effectively locates the positions of the expansion part of the tube, and the compression portion. The remaining portion of the exhaust tube downstream from the expansion chamber has little effect on the xe2x80x9ctuningxe2x80x9d of the exhaust.
Some exhaust mufflers, which are also commonly called xe2x80x9ctuned pipesxe2x80x9d or xe2x80x9ctuned exhaust extractorsxe2x80x9d, which are currently available commercially for small two-stroke engines are sufficiently xe2x80x9ctunedxe2x80x9d to allow optimum scavenging of exhaust from the cylinder of the engine and a charge-densification of the incoming fuel/air mixture. This occurs by the advantageous utilization of the above-described impulse/compression wave nature of the exhaust muffler. There are also mass effects involved in exhaust processes, i.e., the volume of exhaust gas in a system does not move through the pipe with a smooth, linear velocity. The velocity rises and falls along with the pressure waves, so that being xe2x80x9cin tunexe2x80x9d with these differences amplifies the pressure differences. The expansion portion of the exhaust gas wave moving out of the cylinder, through the exhaust valve, and down the muffler tube serves to establish a subnormal pressure condition just outside the exhaust valve, which aids in removing additional combustion products from the cylinder while the cylinder interior is exposed to the open exhaust port. Shortly thereafter, the compression wave passing back up the muffler to the cylinder serves to xe2x80x9csuperchargexe2x80x9d the incoming fuel/air charge that has begun to exit the open exhaust port by forcing the charge back through the exhaust port and into the cylinder, thereby increasing the density of the fuel/air mixture in the cylinder before the compression and combustion cycles are achieved.
Unfortunately, prior art muffler devices for small two stroke gasoline engines offer chamber designs that are many times longer than the diameter of the cylinder in which the fuel/air mixtures are combusted. The most prevalent of such muffler devices commercially available for two-stroke gasoline engines suffers from having a length as much as 6-30 times the diameter of the cylinder it is attached to. The specific length of the tuned pipe is primarily a function of the RPM at which the engine designer wishes to xe2x80x9ctunexe2x80x9d the system. Often a particular torque curve is desired for an optimum match-up with the particular airframe chosen, and this can be achieved by designing the system to be longer or shorter. A short length tube will be utilized for a high-RPM, low torque engine, and a long length tube will be used for a low-RPM, high torque engine. This length is used to create the compression/expansion wave actions referred to above which establish the scavenging and densification functions previously described. If such muffler chamber is not properly sized, the two-stroke engine exhaust will not be xe2x80x9ctunedxe2x80x9d and performance of the engine will suffer drastically.
However, when the muffler chamber is properly sized for optimum performance, it results in a muffler having a physical presence that is many times larger than the entire engine to which it is attached. In the world of small engines, this is very undesirable for several reasons. One reason that such bulky and cumbersome exhaust device is undesirable is the ugly aesthetics that it presents. The present commercially available muffler is a long, cigar-shaped tube that must extend down the side of the vehicle to which it is attached. For those who desire authenticity in the appearance of their small gasoline-powered vehicles, the presence of such a bulky and obvious attachment, often extending down the full length of the airplane or land vehicle on which it is used, greatly mars the owner""s enjoyment of the vehicle. This is particularly true in the field of radio-controlled (RC) airplanes and cars.
In addition to the aesthetically unpleasant feature of current muffling devices, they also are very aerodynamically inefficient, causing unbalanced weight and drag on the vehicles, especially on the RC airplane.
Commercially available xe2x80x9ctunedxe2x80x9d mufflers for small two-stroke engines generally comprise a long, cigar-shaped tube/chamber combination that begins with a small diameter next to the exhaust port of the engine cylinder. At this point the cross-sectional area of the muffler may be approximately the same size as the exhaust port of the small engine. As you progress down the tube of the muffler, the cross-sectional area increases several-fold to form the expansion chamber of the muffler to create the expansion wave which moves backward down the muffler to the exhaust port and provides the scavenging function mentioned previously. This serves to xe2x80x9csuckxe2x80x9d the remaining exhaust gases from the cylinder while also creating a low-pressure condition in the cylinder that aids in inducting a greater fuel/air charge through the intake port which is open at the same time.
Further down the muffler, the cross-sectional area is narrowed significantly to create the compression wave that then moves back up the muffler to the cylinder and serves to push back into the cylinder the portion of unburned fuel/air charge that had managed to flow partly out the exhaust port and into the muffler, thereby accomplishing the densification or xe2x80x9csuperchargingxe2x80x9d effect of the muffler. When these two chamber sections, i.e. the expansion chamber and the compression chamber, are located in tandem along the same axis, the device must by necessity be very long, i.e., many times the diameter of the cylinder to which it is attached. This creates a muffler system that is sometimes longer than the vehicle on which it is used. This prior art exhaust device can thus be characterized as a Total-Axial-Flow muffler system.
A second prior art muffling device that is commercially available is similar to the xe2x80x9ctuned pipexe2x80x9d system described above but adds a further element of a concentric annular outer shell which xe2x80x9cwrapsxe2x80x9d around the tuned muffler and goes from the exit end of the first exhaust pipe, forward to the beginning of the inner pipe to obtain a dual concentric pipe effect. This creates an outer chamber around the inner tube, which chamber serves to act as an expansion/compression wave generating chamber. While this has the effect of providing the desired scavenging and densification effects on the engine and is shorter in length, this second device suffers from the disadvantage of being larger in diameter and less efficient than just the tuned pipe style of muffling system, thus detracting from the aesthetics and streamlining of the vehicle it is used on.
The present invention solves the problems of the prior art exhaust systems described above by providing a muffler device that is not total-axial-flow with respect to exhaust gases, but instead folds the several distinct functional aspects of the xe2x80x9ctunedxe2x80x9d muffler in on one another in a combined axial-circumferential flow, to greatly reduce the length and size of the device and thereby provide a muffler that can be secured entirely inside the cowling, cabin, or cockpit of the vehicle on which it is used. Thus the present invention presents a xe2x80x9ctunedxe2x80x9d muffler that is aesthetically pleasing and which eliminates aerodynamic drag on the vehicle.
The invention also teaches a tuned exhaust system for an internal combustion engine having at least one exhaust port for ejecting spent exhaust gases, said tuned exhaust system comprising:
A. a first enclosed exhaust flow channel adapted for attachment to an internal combustion engine, said first flow channel having at one end thereof an inlet port adapted for receiving exhaust gases from the exhaust port of an engine, an extended flow tube coiled in a first radial plane containing said inlet port, and an outlet port at the opposite end of said flow channel; and,
B. an expansion channel attached to said first flow channel and having an extended chamber folded into a second radial plane axially displaced from said flow channel, and having an inlet opening communicating with said flow channel outlet port, and an exhaust opening located an extended distance down said expansion channel from said inlet opening and adapted to exhaust gas flow into the atmosphere.